Abstract

The lack of through-going, right-lateral surface rupture accompanying the Loma Prieta earthquake has raised questions about the behavior of the San Andreas fault in the southern Santa Cruz Mountains. We re-evaluated previously published reports of ground deformation due to the 1906 earthquake in this region to determine if San Andreas fault surface rupture accompanied this event and whether any of the reported values of offset are reliable. In addition, our search for historical data has brought to light new information about the 1906 earthquake in the southern Santa Cruz Mountains. None of the reports provides indisputable evidence for 1906 San Andreas fault surface rupture. However, taken together with a study of the geomorphology of the fault zone and relationships exposed in excavations across the fault, they do provide a basis for believing that surface rupture extended through the region in 1906. None of the reported values for 1906 offset is a reliable measurement, however, calling into question current and pre-1989 seismic-hazard assessments for the Loma Prieta area. The geologic and geomorphic relationships indicate that shallow, right-lateral crustal strain is typically expressed as surface rupture across a narrow San Andreas fault zone, in contrast to the 1989 Loma Prieta earthquake.

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